1. Field of the Invention
The present invention relates to telecommunications cables. More particularly, the present invention relates to water blocking gels for optical fiber telecommunications cables that are compatible with optical fiber cable buffer tubes made from polyolefin materials.
2. Description of the Prior Art
Optical fiber cables have been used by the telecommunications industry for a number of years to transmit information at very high rates over long distances. Optical fiber cables come in a variety of configurations, including: cables with a centrally located single buffer tube containing one or more optical fibers; cables with a plurality of buffer tubes stranded in a helical or alternating helical arrangement about a central strength member; and cables with slotted cores in which a plurality of optical fibers reside.
In the cable varieties in which the optical fibers are housed in a buffer tube, the buffer tubes' primary function is to protect the delicate optical fibers housed therein. Accordingly, control of the modulus, percent elongation to break, coefficient of thermal expansion, shrinkage, swelling and other physical properties of the buffer tubes is very important. Until recently, polyolefin materials, although having many desirable properties such as low temperature flexibility, low cost and good processability, were not used for optical fiber cable buffer tubes because such materials had a reduced modulus relative to other engineering resins such as polybutylene terepthalate (PBT). However, recent advances, as disclosed in U.S. Pat. No. 5,574,816, to improve the modulus and compression resistance of polyolefin materials, such as low cost impact modified nucleated isotactic polypropylene (i-PP), have made such materials useful for buffer tubes for optical fiber cables.
Although materials like i-PP can enable the manufacture of low cost buffer tubes, such material is not compatible with many traditional, low cost polyolefin based water blocking compounds. The incompatibility is caused by favorable interactions between polyolefin based water blocking gels and the i-PP buffer tube material. As an alternative, polyol based water blocking gels have been developed which offer better compatibility because such materials have a less favorable interaction with the i-PP material. See U.S. Pat. Nos. 5,348,669, 5,433,872 and 5,672,640. Polyol based gels are generally more compatible with water and therefore allow greater amounts of water to be absorbed into the gels. However, as compared to the traditional polyolefin based water blocking gels which have been used in the past with PBT based buffer tubes, the polyol gels may have a greater cost, and therefore, they can reduce the cost savings realized by the use of polyolefin based buffer tubes.
In addition to cost savings, interactions with additives such as anti-oxidant compounds are also an important consideration when determining whether a water blocking gel is compatible. Non-compatible gel materials can swell buffer tube polymers and are able to extract the additives therein and reduce the thermo-oxidative stability of the buffer tubes. Haslov, P. and S. K Ovgaard, "Degradation of PP in Jelly-Filled Cables, IWCS Proceedings (1993), pp. 65-70 teaches that antioxidants can be leached out of the polymer system especially if they are supersaturated.